1. Field of the Invention
This invention relates to a superposition processing method and apparatus for a radiation image, wherein two image signals, which have been independently detected from opposite surfaces of a radiation image storage panel, are superposed one upon the other.
2. Description of the Prior Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or a radiation image storage panel). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
Techniques for carrying out superposition processing on radiation images have heretofore been disclosed in, for example, U.S. Pat. No. 4,356,398. In general, radiation images are used for diagnoses of illnesses and for other purposes. When a radiation image is used for such purposes, it is required that even small differences in the radiation energy absorption characteristics among structures of an object can be detected accurately in the radiation image. The extent, to which such differences in the radiation energy absorption characteristics can be detected in a radiation image, is referred to as the contrast detection performance or simply as the detection performance. A radiation image having better detection performance has better image quality and can serve as a more effective tool in, particularly, the efficient and accurate diagnosis of an illness. Therefore, in order for the image quality to be improved, it is desirable that the detection performance of the radiation image may be enhanced. Practically, the detection performance is adversely affected by various noises.
For example, in radiation image recording and reproducing systems using stimulable phosphor sheets, it has been found that the noises described below occur during the step for recording a radiation image on a radiation image storage panel and reading out the radiation image therefrom.
(1) A quantum noise of radiation produced by a radiation source.
(2) A noise due to nonuniformity in how a stimulable phosphor coated on the radiation image storage panel is distributed or how stimulable phosphor grains are distributed on the radiation image storage panel.
(3) A noise of stimulating rays, which cause the radiation image storage panel to emit light in proportion to the amount of energy stored thereon during its exposure to radiation.
(4) A noise of light, which is emitted by the radiation image storage panel, guided and detected.
(5) An electric noise in the system for amplifying and processing an electric signal.
Superposition processing is carried out in order to reduce the aforesaid noises markedly so that even small differences in the radiation energy absorption characteristics among structures of an object can be found accurately in a visible radiation image, which is reproduced finally, i.e. the detection performance of the radiation image can be improved markedly. Ordinary techniques and effects of the superposition processing are as described below.
Specifically, radiation images are recorded on a plurality of recording media, which overlap one upon another. A plurality of image signals are detected from the plurality of the recording media and then superposed one upon another (i.e., added to one another). In this manner, various noises described above can be reduced. Specifically, an image signal having a high signal-to-noise ratio can be obtained.
Also, it is possible to employ a technique, wherein two image signals are detected from the opposite surfaces of the radiation image storage panel, and the image signal components of the two image signals are then added together, which image signal components represent corresponding picture elements on the front and back surfaces of the radiation image storage panel. In such cases, the image information, which has been recorded with the radiation absorbed by the portion of the stimulable phosphor layer located on the radiation incidence side, is primarily obtained from the surface of the radiation image storage panel, which surface was located on the radiation incidence side when the radiation image storage panel was exposed to the radiation during the image recording operation. Also, the image information, which has been recorded with the radiation absorbed by the portion of the stimulable phosphor layer located on the side opposite to the radiation incidence side, is primarily obtained from the surface of the radiation image storage panel, which surface was located on the side opposite to the radiation incidence side when the radiation image storage panel was exposed to the radiation during the image recording operation.
When the two image signals detected from the opposite surfaces of the radiation image storage panel are superposed one upon the other, a superposition image signal can be obtained which reflects a larger amount of image information. Specifically, an image signal having a higher signal-to-noise ratio can be obtained.
In cases where a visible image is reproduced from an addition signal, which has been obtained by adding two image signals to each other, noise reduces in the visible image as a whole. Also, the sharpness of the visible image becomes low. Therefore, for example, in cases where an image region of a high spatial frequency is to be used and therefore is required to have a good image quality, signal processing should preferably be carried out by using only the image signal, which is detected from the front surface side of the radiation image storage panel, such that the image may have good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. In cases where an image region of a low spatial frequency is to be used and therefore is required to have a good image quality, it is not necessary for the region of a high spatial frequency to have high sharpness in the reproduced image, and noise in the image as a whole should be reduced such that the image may have good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. Therefore, in such cases, the superposition processing described above may be carried out.
As described above, the spatial frequency of interest in the image to be used varies for different kinds of images. Therefore, if the superposition processing is merely carried out, the problem will occur in that appropriate addition processing cannot be carried out. Also, noise components vary for different doses of radiation irradiated to the radiation image storage panel and for different kinds of the radiation image storage panels. Therefore, the addition ratio, in which the two image signals are added to each other and which yields the highest signal-to-noise ratio, varies for different doses of radiation and different kinds of the radiation image storage panels.